Abstract
The European flat oyster Ostrea edulis and the stony sea urchin Paracentrotus lividus play key ecological roles in Mediterranean coastal ecosystems, supporting biodiversity and ecosystem functions. The decline of their natural populations has prompted several restoration initiatives; however, the rapid spread of the invasive blue crab Callinectes sapidus raises concern about its potential impact on native species and the success of these efforts. This study investigated predation by male and female C. sapidus on different size classes of O. edulis and P. lividus under controlled laboratory conditions. Crabs were offered single or mixed prey species, and consumption was monitored over 24 h. Small (~33 mm shell length) oysters were heavily preyed upon (>62%), whereas medium (~60 mm) and large (~82 mm) individuals were not consumed, although attempted predation on medium individuals was observed. In contrast, P. lividus was only occasionally consumed (<32%) at the smallest size (~13.5 mm diameter). Predation differed slightly between sexes at shorter exposure times but converged after 24 h. These results indicate that C. sapidus poses a serious threat to O. edulis restoration, particularly for small individuals, while its impact on P. lividus appears limited. Incorporating predator assessment and mitigation strategies into restoration planning should become standard practice to strengthen the resilience and effectiveness of Mediterranean restoration efforts.